1. Field of the Invention
The invention relates to a polymer, which contains
from 50 to 99.98% by weight of a C.sub.1 -C.sub.12 alkyl (meth)acrylate a), PA1 from 0.02 to 10% by weight of a vinylaromatic compound b), PA1 from 0 to 10% by weight of an ethylenically unsaturated acid or of an ethylenically unsaturated acid anhydride c), and PA1 from 0 to 40% by weight of further monomers d), the percentages by weight being based on the polymer and the polymer being obtainable by polymerizing the monomers a) to d) in at least two stages which differ in their content of vinylaromatic compounds in such a way that the content of vinylaromatic compounds in the vinylaromatics-rich stage is at least 5 times as great as in the low-vinylaromatics stage. PA1 from 50 to 99.98% of a C.sub.1 -C.sub.12 alkyl (meth)acrylate a) by weight PA1 from 0.02 to 10% of a vinylaromatic compound b) by weight PA1 from 0 to 10% by of an ethylenically unsaturated acid c) weight PA1 from 0 to 40% by of further monomers d), which are different weight from a) to c).
Furthermore, the invention relates to the use of the polymers or of their aqueous dispersions as pressure-sensitive adhesives.
2. Description of the Background
Pressure-sensitive adhesives (PSAs) form a permanent tacky film which at room temperature, even under slight pressure, sticks to a very wide variety of surfaces. Pressure-sensitive adhesives are used to produced self-adhesive products such as self-adhesive labels, tapes and films. Products of this kind are very simple to use and make it possible to work rapidly when bonding. In contrast to contact adhesive compositions, no ventilation times are necessary. Moreover, there is no "open time" within which the adhesive bond must be implemented. The quality of a self-adhesive article depends essentially on whether the cohesion and the adhesion of the film of adhesive to the surface that is to be bonded are in tune with one another in accordance with the application.
In the case of pressure-sensitive adhesives for labels, in particular, the level of cohesion must be sufficient for no stringing and no emergence of glue at the edges to occur in the course of stamping and cutting, since otherwise the cutting tools become soiled and the cut faces sticky. At the same time, the adhesion should be at a high level in order to provide good sticking on the substrate that is to be bonded.
In general it is impossible to optimize adhesion and cohesion independently of one another. There is a desire for measures which either lift the level of both properties, or at least maintain one property unchanged while improving the other.
Adhesives, including pressure-sensitive adhesives, based on polymers which can contain styrene are already known from DE-A-3 013 812. U.S. Pat. No. 5,416,134 and EP-A-625 557. The polymers prepared or used in the examples of the disclosures, however, contain at least 4% by weight of styrene. Copolymers of this kind still do not have the desired level of adhesion and cohesion or the desired ratio of adhesion to cohesion.
U.S. Pat. No. 4,939,190 and EP 215241 disclose pressure-sensitive adhesives comprising polymers of multistage construction.